1. Field of the Invention
The present invention relates to a system for controlling energy state aspects of a hybrid vehicle (of the type having an electric motor and some other fuel consuming prime mover) and, more particularly, the present invention relates to a system for monitoring and adjusting a "total energy level" for the hybrid vehicle such that energy storage conditions for providing drive energy to the electric motor are optimized and fuel consumption for prime mover is reduced.
2. Related Art
A conventional series hybrid vehicle may use an electric motor to provide mechanical drive power to the wheels and an internal combustion engine as a prime mover (i.e., an internal combustion engine that drives an alternator or generator, which, in turn, produces electrical energy that is stored and later used by the electric motor). A conventional control system for such a hybrid vehicle commands the electric motor and internal combustion engine to various operating speeds and torques to achieve desired drive power. The control system of the hybrid vehicle is also responsible for managing power flow to and from an electrical energy storage unit for storing the electrical energy used (or produced) by the electric motor. The electrical energy storage unit is usually a battery pack (e.g., a plurality of series/parallel connected batteries, such as lead acid batteries, nickel cadmium batteries, nickel metal hydride batteries, lithium batteries, etc.). Recharging the battery pack while operating the hybrid vehicle may be accomplished in any number of ways, for example: (i) using a generator and passive regulator, (ii) using a generator and an active recharging power converter, (iii) using an alternator and passive rectifier/regulator; and (iv) using an alternator and an active recharging inverter (AC to DC converter). It is understood that the above list is given by way of example and is not intended to be exhaustive. In any case, the generator or alternator is rotatably coupled to the internal combustion engine such that the generator or alternator is capable of producing a source of electrical power to recharge the battery pack in response to the rotational power provided by the internal combustion engine.
The conventional control system often monitors the state of charge of the battery pack and maintains the state of charge at approximately 60% of full charge. It is understood that the battery pack state of charge is not a tightly regulated quantity, rather, the control system provides course regulation to substantially within some range centered at about 60% state of charge. Often, the 60% state of charge results in: (i) a battery pack impedance which is relatively low (e.g., for lead acid batteries); (ii) a battery pack having a source of reserve energy capable of providing drive power for unforeseen upward grades (e.g., hill climbing) and/or acceleration, it being understood that power is the time rate of change of energy; and (iii) a battery pack having a charge space sufficient for recovering mechanical potential and/or mechanical kinetic energy from the hybrid vehicle during downward grades and/or deceleration (e.g., where the electric motor operates in a regenerative mode).
When the state of charge falls below the 60% level by a sufficient amount, the control system will cause the internal combustion engine and generator or alternator apparatus (hereinafter the "generator" for simplicity) to engage and provide electrical charging power to the battery pack. This is often accomplished by increasing the commanded speed and/or torque of the internal combustion engine. As the generator delivers electrical power to the battery pack, the state of charge of the battery pack increases. When the state of charge exceeds the 60% level by a sufficient amount, the control system will cause the internal combustion engine and generator to terminate delivery of electrical recharging power to the battery pack.
This control protocol for battery pack state of charge provides adequate results when operating over flat terrain, but becomes less satisfactory when the terrain includes a substantial number of upward and/or downward grades of substantial length. For example, when the hybrid vehicle is operating at a relatively high velocity and/or at a relatively high altitude, the probability that kinetic and/or potential energy from the hybrid vehicle may be recovered and stored in the battery pack is higher than when the hybrid vehicle is operating at relatively low velocities and/or low altitudes. Stated another way, a hybrid vehicle which has increased its velocity and/or climbed a substantial hill (increased its altitude) will usually reduce velocity and/or altitude at some point. This represents an opportunity to recover mechanical energy from the hybrid vehicle for recharging the battery pack.
Unfortunately, the prior art control systems are unable to account for mechanical kinetic and/or mechanical potential energies of the hybrid vehicle in the above-noted circumstances and, therefore, do not enjoy the advantages of efficient recovery of this energy. Further, these conventional control systems are incapable of anticipating upward and/or downward grades on which the hybrid vehicle may travel. Thus, adjustments in the desired state of charge of the battery pack cannot be made to optimize the state of charge of the battery pack for future energy demand and/or charge space requirements.
Accordingly, there is a need in the art for a new control system for hybrid vehicles capable of adjusting at least the energy state of an energy storage unit in a way which optimizes performance of the hybrid vehicles, particularly on upward and/or downward grades.